1. Field of the Invention
The present invention relates to a metal-supported solid oxide fuel cell and a manufacturing method thereof. More specifically, the present invention relates to a metal-supported solid oxide fuel cell with markedly improved durability and sealing efficiency in which a metal-supported unit cell can be securely sealed to a separator by welding to prevent leakage or mixing of a fuel gas and air prior to reactions in the unit cell and in which the fuel gas and air are supplied through respective defined flow passages to achieve high energy production efficiency, and a method for manufacturing the metal-supported solid oxide fuel cell.
2. Description of the Related Art
Fuel cells are devices that directly convert chemical energy produced by oxidation of fuel into electrical energy. Fuel cells are new environmentally friendly future energy technologies that generate electrical energy from abundant substances such as hydrogen and oxygen on the earth.
A typical fuel cell includes an air electrode as a cathode to which oxygen is supplied and a fuel electrode as an anode to which hydrogen is supplied. In the fuel cell, the oxygen and the hydrogen undergo electrochemical reactions as reverse reactions of water electrolysis to generate electricity, heat and water. As a result, the fuel cell produces electrical energy with high efficiency without causing environmental pollution.
Such fuel cells are free from the limitations of the Carnot cycle, which acts as a factor limiting the efficiency of conventional heat engines, resulting in a 40% or higher increase in efficiency. Fuel cells discharge water only, posing no risk of environmental pollution. Further, fuel cells possess many advantages of size reduction and no noise production because they include no mechanically moving parts, unlike conventional heat engines. Based on these advantages, much research on fuel cell technologies is actively underway.
Fuel cells are classified into phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), polymer electrolyte membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs) and alkaline fuel cells (AFCs) by the kind of electrolyte that they employ. The six types of fuel cells are available or are currently being developed in the art. The characteristics of the respective fuel cells are summarized in the following table.
TABLE 1TypePAFCMCFCSOFCPEMFCDMFCAFCElectrolytePhosphoricLithiumZirconia/ceria-Hydrogen ionHydrogen ionPotassiumacidcarbonate/basedexchangeexchangehydroxidepotassiummembranemembranecarbonateIonHydrogen ionCarbonate ionOxygen ionHydrogen ionHydrogen ionHydrogenconductorionOperation200650500-1,000<100<100<100temp.(° C.)FuelHydrogenHydrogen/carbonHydrogen,HydrogenMethanolHydrogenmonoxidehydrocarbon,carbon monoxideFuel materialCity gas,City gas, LPG,City gas, LPG,Methanol,MethanolHydrogenLPGcoalhydrogenmethane,gasoline,hydrogenEfficiency 40 4545 45 30 40(%)Output range100-5,0001,000-1,000,000100-100,0001-10,0001-1001-100(W)ApplicationDistributedLarge scaleSmall/middle/largePower source forPortablePowerpowerpowerscale powertransportationpower sourcesource forgenerationgenerationgenerationspaceshipDevelopmentDemonstrated-Tested-Tested-Tested-Tested-Applied tostageactually useddemonstrateddemonstrateddemonstrateddemonstratedspaceship
As can be known from the table, the fuel cells can be suitably selected according to the intended purpose taking into consideration their output ranges and applications. Of these, the solid oxide fuel cells (SOFCs) are advantageous in that it is relatively easy to control the position of electrolytes, there is no risk that electrolytes may be used up because the electrolytes are fixedly positioned, and the life of constituent materials is long due to the weak corrosiveness of electrolytes. Based on these advantages, the solid oxide fuel cells have drawn a great deal of attention for use in distributed power generators and in commercial and household applications.
According to the operational principle of a general solid oxide fuel cell, when oxygen is supplied to an air electrode and hydrogen is supplied to a fuel electrode, the following reactions occur in the respective electrodes.
Reaction in the fuel electrode (Anode): 2H2+2O2−→2H2O+4e−.
Reaction in the air electrode (Cathode): O2+4e−→2O2−
The solid oxide fuel cell uses YSZ (yttria-stabilized zirconia) as an electrolyte, a Ni-YSZ cermet as a fuel electrode, a perovskite material as an air electrode, and oxygen ions as mobile ions.
FIG. 1 is a schematic view of a prior art solid oxide fuel cell 1. The solid oxide fuel cell 1 includes: a unit cell 10 including a fuel electrode 12, an air electrode 13, and an electrolyte layer 11 interposed between the fuel and air electrodes; current collectors 20 provided on both surfaces of the unit cell 10; and lower and upper separators 30a and 30b accommodating the unit cell 10 and the current collector 20 therein.
The separators 30a and 30b support the unit cell 10 and the current collectors 20. The separators 30a and 30b have supply passages 31a and 31b through which a fuel gas and air (oxygen) are supplied, respectively.
The fuel gas and air must flow in the solid oxide fuel cell 1 only through the defined passages. Mixing or leakage of the fuel gas and air considerably deteriorates the performance of the fuel cell 1, and there is thus a need for a highly advanced sealing technique to increase the performance of the fuel cell 1.
In the solid oxide fuel cell 1, glass-based sealing materials 40 are generally used to join the separators 30a and 30b and join the unit cell 10 to the separators 30b (In FIG. 1, the air electrode 13 is joined to the upper separator 30b by one of the sealing materials 40).
However, the glass-based sealing materials 40 do not have sufficiently high strength required in the solid oxide fuel cell 1 because they tend to be broken by an external impact. Further, the glass-based sealing materials 40 are readily deformed due to a repeated temperature change, making it difficult to expect sufficient sealing ability. These problems are main causes leading to deterioration in the performance of the solid oxide fuel cell 1.
The current collectors 20 are arranged between the unit cell 10 and the separators 30a and 30b to improve the electrical performance of the fuel cell 1. The current collectors 20 are in the form of a mesh made of a metal alloy or noble metal such that the fuel gas and air are uniformly supplied to the unit cell 10. However, the mesh type structure of the current collectors 20 renders the sealing of the fuel cell 1 more difficult.
To attain a sufficient voltage from the unit cell 10 as an only module, there is a need to increase the area of the unit cell 10 or laminate another unit cell on the unit cell 10 to form a stack. However, the requirements of mechanical strength and sealing performance are more difficult to meet.